Manipulator for automatic test equipment with active compliance

ABSTRACT

A manipulator for an automatic test equipment test head. The manipulator includes a plurality of motors that are coupled with load cells. A controller provides motor control signals based on the output of the load cells. The manipulator may be operated in compliant mode in which the motors drive the manipulator to move in response to external forces. The manipulator can be used in conjunction with a docking system in which a mechanical interface between the test head and the handling device defines the position of the test head relative to the handling device.

This invention relates generally to manipulators for test heads and morespecifically to manipulators which are used in positioning test headsthat are docked to handling devices.

Semiconductor test equipment is often constructed with a test head thatcontains electronic circuitry that must be very close to the deviceunder test. Physical closeness is particularly important when a longcable between the device under test and the electronic circuitry makingtest measurements would create inaccuracies. Consequently, muchcircuitry is placed in the test head of a semiconductor test system. Thetest system might weigh literally hundreds of kilograms. The cableconnecting the test head to the rest of the test system can also belarge, weighing over 100 kilograms.

Even though the test head is very large, it must still be movable. Thetest head is usually connected to a handling device which presents tothe test head semiconductor components on wafers or as packagedcomponents. The test head must be moveable so that it can interface, or"dock" with the handling device. Not only must the test head be movableto be near the handling device, it must also be movable into differentorientations so that the test points on the test head align with thesemiconductor components in the handling device.

For example, some handling devices operate in a "DUT up" configuration.In that configuration, the DUT, or device under test, is on the uppersurface of the handling device and the test head rests on top of thehandling device. Other handling devices operate in a "DUT down"configuration. In that configuration, the test head is rotated 180degrees from the DUT up configuration and connects to the bottom side ofthe handling device. There are also other possible configurations, suchas DUT vertical.

A test system is generally designed to interface with many differenttypes of handling devices. In order to get the very heavy test head intoone of a number of different positions, the test head is held in amanipulator. The manipulator supports the test head and can be moved,often with six degrees of freedom, to put the test head in almost anyconfiguration. Many manipulators are motorized to make it easier for ahuman to move the heavy test head.

In addition, the test head must often be moved after it is docked to thehandling device. Sometimes, the test head must be moved to change afixture or to access the handling device for service. A difficulty inmoving the test head after it has been docked is that it often takes along time to re-dock the test head. Modern semiconductors are so smalland contain so many test points that great care must be taken inaligning the semiconductor under test with the test system. Afterdocking, a calibration routine is often performed by the tester and thehandling device to determine the relative position of the test systemand the handling device. This calibration routine can be performed invarious ways. For example, calibration might be done with a camera onthe handling device that can recognize the test probes on the test head,thereby determining the relative position of the test head and thehandling device. Alternatively, calibration might be done by moving asample device in the handling system until electrical contact is madebetween the sample device and the test system. By detecting the positionat which contact is made, the relative position of the test head and thehandling device can be determined. Once the position of the test headrelative to the handling device is determined, the handling device canadjust the position of the semiconductor device to ensure that thesemiconductor connects with the test system.

Regardless of which method of calibration is used, the amount of timerequired for calibration might be quite long. It is highly desirable tonot have to repeat the calibration process each time the test head ismoved. The problem with a conventional docking arrangement is that,after the test head is moved, it often does not return to exactly thesame docking position. "Play" in the mechanical interface between thetest head and the handling device means that the parts will not returnto exactly the same position after they are moved. Additionally, thecable attached to the test head, because it is so heavy, can influencethe orientation of the test head. When the test head is moved, the cablerarely returns to the same spot. Thus, the forces applied by the cablechange whenever a test head is moved, making it less likely that thetest head will return to the same spot.

One innovative solution has been implemented by Teradyne, Inc. Thesolution, called the K-dock™ interface, involves the use of a mechanicalinterface between a test head and a handling device that uses contactsshaped to provide limited docking positions. Whenever the system isdocked, the test head returns to the same relative position as thehandling device. This docking system is described in U.S. patentapplication Ser. No. 08/463,227 by Slocum et al., which is herebyincorporated by reference.

With this approach, the final positioning of the test head is dictatedby the mechanical interface rather than the settings of the manipulator.For this to work, "compliance" must be built into the manipulator."Compliance" means that, at controlled times, the manipulator does notrigidly hold the test head in place. A manipulator with mechanicalcompliance is shown in U.S. Pat. No. 08/808,131 filed Feb. 28, 1997 bySlocum, et al., which is hereby incorporated by reference.

The manipulator in the aforementioned application Ser. No. 08/808,131 isa six degree of freedom manipulator. Various mechanical methods wereused to provide compliance. For example, in some directions, compliancewas provided by simply releasing a brake that allowed a part of themanipulator to move. As another example, compliance was provided in thevertical direction by an air cylinder that could be compressed. In otherdirections, the motors were spring biased such that they could move asmall amount if pressed.

I have discovered certain ways in which this design can be improved.First, the mechanical methods of providing compliance are oftensensitive to test head weight. For example, a spring or air cylindershould provide a force to hold the test head in the middle of itstravel. If the test head is heavier or lighter than the one for whichthe manipulator was designed, the test head will likely wind up at oneend of its travel. Therefore, the amount of compliance will be limited.

A similar problem exists if the test head position changes. The forceexerted by the test head on various joints or bearings of themanipulator will depend on test head position. The required complianceof those joints will therefore change as the test head position changes.

It would be highly desirable if the compliance of the manipulator werenot sensitive to the weight or orientation of the test head or cablesattached to the test head.

SUMMARY OF THE INVENTION

With the foregoing background in mind, it is an object of the inventionto provide a manipulator for holding a test head for a semiconductortest system.

It is also an object to provide an improved manipulator for compliantdocking.

The foregoing and other objects are achieved in a manipulator having atleast one motor driving motion of the test head. A load cell measuresforces on the test head. During one operating mode, the output of theload cell is not used to control the motor. However, in compliant mode,the output of the load cell is used to control the position of the testhead such that the force on the test head remains constant.

In one embodiment, the range of drive of the motor is limited.

In yet another embodiment, the range of force on the load cell islimited and the motor is shut down if the force exceeds a predeterminedlevel.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be better understood by reference to the followingmore detailed description and accompanying drawings in which

FIG. 1 illustrates a manipulator according to the prior art;

FIG. 2 is a block diagram of a manipulator incorporating a load cellaccording to the invention;

FIG. 3A illustrates a flow chart of the control program during a normalmode of operation for a manipulator incorporating the invention; and

FIG. 3B illustrates a flow chart of the control program during acompliant mode of operation for a manipulator incorporating theinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows a manipulator 102 holding a test head 108. In use, a cable(not shown) would typically be used to attach test head 108 to a testerbody (not shown).

Manipulator 102 is made from various assemblies that allow test head 108to be moved with multiple degrees of freedom. Test head 108 is mountedin cradle assembly 600. The mountings allow test head 108 to move in thenod direction, indicated as NU and ND.

Each mounting is on a carriage 610 that can slide along the arm. Thecarriage 610 in each arm can be moved independently. If the carriagesmove in the same direction, test head 108 moves in the In or Outdirection, indicated as I or O in FIG. 1. However, of the carriages 610are moved in opposing directions, test head 108 will rotate in thedirections indicated as ΘL or ΘR.

Cradle assembly 600 is attached to a bearing assembly 500. Bearingassembly 500 allows rotation in the direction indication RL or RR.

Bearing assembly 500 is attached to a telescoping shaft assembly 400.Shaft assembly 400 allows motion in the Up and Down direction, indicatedas U and D.

Shaft assembly 400 is mounted on a base assembly 300. Base assembly 300includes a linear bearing that allows translation in the directionsindicated L and R. Base assembly 300 also includes a rotary bearing thatallows rotation in a direction indicated as SL or SR.

Each motion might be motor driven to make positioning of test head 108easier. For example, carriages 610 could each be attached to a motor.Likewise, shaft assembly 400 likely contains a motor. The motors arecontrolled by electronics in control unit 310. Control unit 310 containsmotor control circuitry as known in the art. Examples of known controlcircuitry are microprocessors, micro-controllers and programmable logiccontroller. All of these devices are commercially available fromnumerous sources.

A human operator can enter commands to the control unit 310. Thecommands indicate when and the direction in which test head 108 is to bemoved. Control unit 310 translates command inputs into control signalsthat turn on and off the various motors within manipulator 102 to drivetest head 108 to the desired position.

Command inputs can also be input to control unit 310 from the tester orhandling device with which manipulator 102 is operating. One importantcommand input when the K-dock™ interface is used indicates that themanipulator should be placed in compliant mode to enable docking. Thecontrol program executed by control unit 310 changes when themanipulator is in compliant mode.

The control program in compliant mode is operates on force feedback fromload cells mounted throughout the manipulator. Load cells arecommercially available devices that produce an electrical outputproportional to the force along one axis of the load cell. Examples ofvendors which sell load cells are Omega and SensoTech. The electricaloutput of the load cell serves as an input to control unit 310.

For each direction in which compliant motion is desired, there should beat least one load cell. The load cell should be coupled at one end tothe motor that drives motion in that direction. The load cell should bemounted such that it can measure the force applied by the load--the testhead--on the motor. Several mounting configurations can be used toachieve this result. One configuration is to couple one end of the loadcell to the load and the other end to the motor. Another configurationis to couple one end of the load cell to the motor and the other end toa fixed position. In this configuration, the motor must be allowed tomove a slight amount. A flexure or bearing mounting scheme can be usedto allow the necessary movement. Regardless of mounting configuration,the load cell measures the force applied by the load to the motor in thedirection of travel.

FIG. 2 gives one example of the mounting of a load cell 214. As depictedin FIG. 2, load cell 214 measures force in the direction M in which acarriage 610 moves.

One end of load cell 214 is coupled to motor 212. The other end of loadcell 214 is coupled to rail 216. Carriage 610 slides along rail 216 toprovide motion in the direction M. Thus, rail 216 does not move whencarriage 610 moves in direction M.

Motor 212 drives lead screw 218, which pushes or pulls carriage 610 indirection M. Motor 212 is mounted in bracket 220 that allows limitedmotion of the motor in the direction M, but provides support andrestrains motion in other directions.

Test head 108 (FIG. 1) is connected to carriage 610 by means of shaft222. Shaft 222 might be attached to carriage 610 through a ball joint orother mechanism. A force applied to test head 108 is measured by loadcell 214. Likewise, if the weight of test head 108 applies force oncarriage 610, that force is likewise measured by load cell 214.

Motor 212 has a control input that is connected to controller 310. Thecontrol input is made up of a series of wires through which electricalsignals are carried. These electrical signals control motor 212, in aknown manner. These control inputs allow controller 310 to regulate thedirection and speed of motor 212.

Load cell 214 has a measurement output that is connected to controller310. The electrical signal on that line indicates the force measured byload cell 214. It will be appreciated by one of skill in the art thatwell known elements of an electromechanical system, such as powerconnections, are not expressly shown but are used in conjunction withthe invention.

The number of load cells mounted on manipulator 102 depends on thenumber of directions in which motion will be motorized. Also, it willdepend on the number of directions in which compliant motion will berequired during docking. Preferably, each axis about which motion ismechanized will have a load cell for use in compliant mode.

In a preferred embodiment, there will be one load cell attached to eachcarriage 610 in the cradle assembly. The cradle assembly 600 issymmetrical on each side of test head 108. Therefore, there are two loadcells in cradle assembly 600.

In a preferred embodiment, there will also be a load cell in bearingassembly 500 and telescoping shaft assembly 400. Within base assembly300, there will also be a load cell mounted to measure force in thedirection indicated as L-R.

FIG. 3 illustrates how controller 310 uses the outputs from each of theload cells to control its associated motor. The load cell and motormounted to sense force and provide motion in a particular direction areused together for processing. The output of the load cell is used tocontrol the motor. Each motor, though, is processed independently. FIG.3 shows the processing for one motor. When multiple motors are used, theprocessing shown in FIG. 3 is separately performed for each.

FIG. 3A shows how the load cell might be used during a "normal" mode ofoperating manipulator 102. In normal mode, an operator enters commands,such as through a pendant, joy stick or other similar input device, thatindicate the directions in which test head 108 should be moved. Thus,the process of moving test head 108 begins at step 312 when a commandfrom a human operator is received to move test head 108.

At step 314, controller 310 generates commands that drive the motor. Thedirection in which the motor will be driven depends on the commandreceived.

At step 316, controller 310 reads the value from the load cell. At step318, the value of the load cell is compared to limits. If the limits areexceeded, processing proceeds to an error handler program at step 324.

The error handler at step 324 will, in a preferred embodiment, cause themanipulator to stop. It will also actuate an audio or visual alarm thatalerts the operator to a problem.

The limit used at step 318 will be set to identify a force that shouldnot occur in normal operation. Therefore, the limits will signal animproper, and therefore potentially unsafe, operating condition. Thespecific value of the limit that is used will depend on the specifictest head and manipulator design. Each motor will have its own limits.The limits might, in a preferred embodiment, be different depending onthe direction in which the motor is driving the test head. For example,the force limit when driving the test head in the U direction would belarger than the force limit when driving the test head in the downdirection.

Or, more complicated limits might be programmed based on the orientationof the test head. The amount of force that would be "normal" on a motordriving carriage 610 would be much less with the test head in theposition shown in FIG. 1 as opposed to having the test head rotated 90°in the RL direction. In a preferred embodiment, controller 310 willcontain tables in computer memory that store the limits.

Alternatively, the limits might be computed dynamically. The limit mightbe computed by measuring the output of the load cell before motion isstarted. The limit would then be computed by adding an allowed deviationto that measurement. In this embodiment, the allowed deviation is storedin memory inside controller 310.

Regardless of how the limits are set, if the limits are not exceeded,execution proceeds to step 320. At step 320, a check is made whether theoperator input still indicates more motion of manipulator 102 isrequired. If the operator input is off, no further motion is required,and the process ends at step 332. Commands to the motor stop the motorand lock it in position. When no motion is required, manipulator 102should be as rigid as possible so that test head 108 is held still.

Alternatively, if the operator control is still on, execution returns tostep 314 where the motor is drive further in the required direction.

As described in the above mentioned patent applications, significantperformance advantages in a test system can be achieved if the positionof the test head is not set by the manipulator. Rather, it is desiredthat the position of the test head be set by a mechanical interfacebetween the test head and a handling device. For that to happen, though,the manipulator must be able to enter a "compliant mode". In compliantmode, the test head can be pushed or pulled into the required position,as dictated by the mechanical interface.

Compliant mode operation is illustrated in FIG. 3B. Compliant modeoperation starts at step 350. Step 350 is executed when controller 310receives a command indicating that compliant mode should be entered. Thecommand to enter compliant mode might come from a user.

Alternatively, the command might come from the test head or the handlingdevice or a computer work station controlling both the tester and thehandling device. In a preferred embodiment, compliant mode is notentered until the pieces of the mechanical interface on the test headare latched to the pieces of the mechanical interface on the handlingdevice. After the pieces are latched, they are drawn together to definethe final position of the test head. Thus, in a preferred embodiment, asensor is employed to generate a signal indicating that the pieces ofthe interface have been latched. Compliant mode lasts until the testhead is docked in a position dictated by the components of themechanical interface.

Once it is determined that compliant mode should be entered, executionproceeds to step 352. At step 352, the value of the load cell is stored.As mentioned above, the processing of FIG. 3B is performed once for eachmotor in compliant mode. Preferably, this value is stored in digitalform in a computer memory that is a part of controller 310.

At step 354, the position of the test head is also stored. Positionalsensors are known in the electromechanical are and manipulator 102 couldbe equipped with such sensors. The position of test head 108 in any ofthe directions indicated in FIG. 1 can therefore be determined andrecorded.

At step 356, the load cell value is measured again. If the force on themotor has changed, processing continues at step 360. A change in theforce indicates that the mechanical interface is trying to pull or pushtest head 108 into a different position. However, if the force has notchanged, execution proceeds to step 366.

Where the force on test head 108 has changed, step 360 determineswhether that force has increased or decreased. If the force hasincreased, execution proceeds to step 362 where the motor is drivenclockwise. On the other hand, if the force has decreased, executionproceeds to step 364 where the motor is driven counter-clockwise.

In this way, the motor is driven to ensure that the force in the testhead stays the same. As external devices place force on the test head,the test head is driven in the direction of those forces. In thepreferred embodiment, those external forces are imposed by themechanical interface between the test head and the handling device.

At step 366, a check is made to determine whether the test head and thehandling device are docked. In a preferred embodiment, this stepinvolves checking a test signal that is generated by the mechanicalinterface. However, various other ways could be used to determine thatthe test head has been moved to the required position relative to thehandling device.

Once the test head is docked to the handling device, execution proceedsto step 372. At step 372, compliant mode is ended. Compliant mode isended by sending control signals to the motor that lock it rigidly intoposition.

If step 366 determines that the test head and handling device are notdocked, execution proceeds to step 368. At step 368, one or more checksare made to determine whether compliant mode should be ended before thetest head and the handling device are docked. For example, a limitrepresenting the distance the test head should move in compliant modemight be programmed into control unit 310. Alternatively, compliant modemight be programmed to last for only a certain period of time. Step 368would, for example, check whether limits on time, distance, or otherfactors have been exceeded. If limits have been exceeded, executionwould proceed to step 370, where compliant mode ends. Exceeding limitsmight indicate an error condition. Thus, step 370 might also include anoutput indicating an error condition, such as an audio or visual alarmsignal.

When the test head and handling device are not docked and limits havenot been exceeded, execution returns to step 356. The value of the loadcell is checked again and the test head is moved, in a closed loopfashion, to move the test head in response to the force. It will beappreciated that FIG. 3 shows the steps of a program taken in discretesteps. Such would be the case were the control program implemented as adigital program on a microprocessor. However, the motor and load cellare intended to form a closed loop feedback control system and, what areshown as discrete steps, might be performed essentially simultaneously.

Having described one embodiment, numerous alternative embodiments orvariations might be made. For example, the number and placement ofmotors and load cells can be varied depending on the design of themanipulator. Not every motion needs to be motorized. In a preferredembodiment, load cells will be provided only for carriages 610 in cradleassembly 600 and telescoping assembly 400.

Also, it was described that the load cells were used in normal mode todetermine whether an error condition occurred in addition to be using incompliant mode. These two functions need not be provided in the samemanipulator. However, if the load cells are installed in the manipulatorto provide compliance, very little added cost is imposed to use the loadcells to check for error conditions in normal operation.

In addition, it should be appreciated that FIG. 3B shows the motor isdriven clockwise or counter-clockwise depending on the change in themeasured force. The direction in which the motor must be driven toprovide complaint motion will depend on the way in which the motor isattached to the manipulator.

Also, it was described that compliant mode is used for docking the testhead to the handling device. Compliant mode could be used to allow aperson to position a manipulator by pressing on it. Instead of using apendant to position the test head, a human operator could apply force tothe test head using a set of handles. These forces would be measured bythe load cells in the manipulator. The controller would then control themotors in the manipulator to drive the test head in the direction of theapplied force.

Also, it was described that each motorized axis for which activecompliance was provided was treated independently. In some instances,force applied in only one direction will "couple" to load cellsmeasuring force in different directions. It might, therefore, benecessary or desirable to include in FIG. 3B a force decoupling step.For example, at step 356 where the value of one load cell is measured,the measured value might be corrected by adding or subtracting somefraction of the force measured at another load cell. The fraction wouldbe selected based on the amount of coupling between the two directions.

Further, it was described that load cells are used to measure forces.Other force measuring devices, such as strain gauges, might be used.Likewise, it was described that a lead screw 218 to couple the motor tothe load. Other coupling mechanisms, such as belt drives or ball screws,might be used, depending on the specific design of the manipulator.

Therefore, the invention should be limited only by the spirit and scopeof the appended claims.

What is claimed is:
 1. An automatic test system of the type having atest head and a handling device, comprising:a) a manipulator for thetest head, comprisingi) a base element, ii) a moveable element movablymounted with respect to the base element, iii) a motor, having anelectrical control input, the motor being mechanically coupled to themovable element, thereby driving the movable element with respect to thebase, iv) a load cell having a first end and a second end and anelectrical output producing an electrical signal in proportion to theforce on the load cell in the direction between the first end and thesecond end, at least one end of the load cell being mechanically coupledto the motors wherein a second end of the load cell is coupled to thebase element, and v) a controller having an electrical input connectedto the electrical output of the load cell and an electrical outputconnected to the electrical control input of the motor, the controllerproducing an output that varies in proportion to the input.
 2. Theautomatic test system of claim 1 wherein the moveable element comprisesa carriage to which the test head is coupled and the base elementcomprises a linear rail and a linear bearing between the carriage andthe linear rail.
 3. The automatic test system of claim 1 wherein themoveable element comprises a telescoping shaft.
 4. The automatic testsystem of claim 1 additionally comprising a bearing mounted to the baseelement, said bearing slidably supporting the motor.
 5. An automatictest system of the type having a test head and a handling device,comprising:a) a manipulator for the test head, comprising a plurality ofmotorized mechanisms, each mechanism havingi) a base element, ii) amovable element movably mounted with respect to the base element, iii) amotor, having an electrical control input, the motor being mechanicallycoupled to the movable element, thereby driving the movable element withrespect to the base, and iv) a load cell having a first end and a secondend and an electrical output producing an electrical signal inproportion to the force on the load cell in the direction between thefirst end and the second end, the first end of the load cell beingmechanically coupled to the motor; and b) a controller having aplurality of electrical inputs, each connected to the electrical outputof one of the load cells and a plurality electrical outputs eachconnected to the electrical control input of one of the motors, thecontroller producing output signals that each vary in proportion to theinput from a load cell.
 6. The automatic test system of claim 5 whereinthe moveable element of one of the plurality motorized mechanismscomprises a carriage and the moveable element of a second of theplurality of motorized mechanisms comprises a vertical shaft assembly.7. The automatic test system of claim 5 additionally comprising a leadscrew coupling the motor to the moveable element.
 8. The automatic testsystem of claim 5 wherein the base elements of two of the motorizedmechanisms comprise opposing cradle arms with the test head mountedbetween the cradle arms.
 9. A method of operating an automatic testsystem of the type having a handling device and a test head mounted on amanipulator, comprising the steps of:a) generating a signal indicatingthe test head is latched to the handling device; b) measuring the forceat at least one point in the manipulator as a reference; c) applyingforce on the test head to urge it into a docked position; d) driving amotor within the manipulator to move the test head in a direction thatmakes the force on the test head closer to the reference force; and e)placing the motor in a rigid state when the test head is in the dockedposition.
 10. The method of claim 9 additionally comprising the steps ofmonitoring the motion of the test head and ceasing the step of drivingwhen the amount of motion of the test head exceeds a limit.
 11. Themethod of claim 10 wherein the step of monitoring the motion of the testhead comprises monitoring the distance traveled by the test head. 12.The method of claim 9 additionally comprising the steps of, prior tolatching the test head to the handling device, driving a motor to movethe test head while monitoring the output of the load cell and stoppingdriving the motor when the output of the load cell exceeds a thresholdvalue.
 13. The method of claim 9 wherein the step of measuring the forceat at least one point comprises measuring the force asserted on themotor in the direction in which the motor drives the test head.
 14. Themethod of claim 13 wherein the step of measuring the force includesstoring an indication of the force in memory associated with acomputerized controller.
 15. The method of claim 9 additionallycomprising, prior to generating a signal, the steps of:a) applying aforce to the test head with a human operator; b) driving a motor withinthe manipulator to move the test head in a direction that reduces theforce on the test head.